The present invention relates to infrared test targets used for evaluating the performance of infrared equipment, and more particularly, to a miniature thermal target having a four-bar target in front of an ambient field while maintaining the proper temperature difference between the field and the bars.
Heretofore, differential blackbodies have been used as infrared energy emitting test targets to test and evaluate the performance of infrared equipment. Such infrared equipment is employed in forward looking infrared (FLIR) systems and spacebased staring infrared systems, for example. A typical conventional blackbody simply uses an aperture plate located in front of a uniform plate. The temperature difference between the two plates provides the desired contrast.
A more elaborate type of test target is illustrated by U.S. Pat. No. 4,446,363, issued to Charles T. Lakin, et al., May 1, 1984, and assigned to the United States Government. The patent is for a Target for Optically Activated Seekers and Trackers (TOAST). It provides a target for an optically activated seeker which simulates the three degrees of freedom of an actual maneuvering target. It combines a target spot image with a background image by means of a beamsplitter mirror. An optically activated seeker and tracker is positioned in a three degree of freedom gimbal so as to view the combined target and background image. The gimballed mounting system provides the seeker with all the angular inputs of guided missile flight except for those of acceleration forces. The target is calibrated and can be varied as to size, intensity, spatial position, color and interfering background. This test target supersedes a crude system of mounting a small lamp on a plotting board. The plotting board was positioned ten feet in front of the seeker, using the X and Y motions to simulate target motion. The plotting board system was disadvantageous in that the seeker had to be disassembled and focussed for the test at a 10 foot distance, then disassembled and refocused after the test. In addition, the plotting board target could not provide for changes in target intensity or target size. The plotting board system could not provide a competitive background to test the ability of the seeker to reject interfering targets.
Current requirements make it desirable to have miniature infrared test targets particularly for use as field test sets, such as for testing FLIR systems in the field, for example. Unfortunately, present differential blackbodies having an aperture plate located in front of a uniform plate cannot be miniaturized because, when they are scaled down, the blackbodies have an extremely high temperature gradient. It is impractical to maintain an extremely high temperature gradient in a miniaturized differential blackbody test set. This is because the temperature differences between the aperture plate and the uniform plate provides the desired contrast. When the traditional differential blackbody is scaled down to form a miniature test target, the two plates are closer together, and it becomes necessary to maintain an extremely high temperature gradient. The result is an unstable high power consumption device.
The test target in U.S. Pat. No. 4,446,363 is too elaborate and complicated to be miniaturized, and the features it provides are not needed to test FLIR systems in the field. It is not necessary to simulate an actual maneuvering target, or to have a calibrated target that can be varied in size, intensity, spatial motion, color and background.
Other currently used conventional miniature infrared test targets of the type known as miniature emissivity targets have a problem of a non-blackbody nature that makes the targets very sensitive to the Narcissus effect and to complicated environmental changes. This is due to the nature of the surfaces with which it is made. Furthermore, the emissivity targets have a very limited contrast range, and the target has to be kept at a higher temperature than the preferred real world temperature.
Accordingly, it is an objective of the present invention to provide a miniature thermal target having a 4-bar target combined with a target field in which the temperature difference between the field and the bars is proportional to the true temperature difference. Another objective of the invention is the provision of a miniature infrared test target that overcomes the difficulty of maintaining an extremely high temperature gradient in the target. Still another objective of the present invention is to provide a miniature thermal target having a greater contrast range than that provided by conventional miniature targets. Yet another objective of the invention is the provision of a miniature infrared test target that is made with surfaces approximating a blackbody, thereby providing immunity from the Narcissus problem and the effects of complicated environmental changes.